It is known that glasses whose surfaces are darkened may be used to prepare many different products. Thus, for example, a surface-darkened glass in the form of a disc may have its faces polished so that they transmit light through the visible region of the spectrum but have its perimeter substantially absorb such light. The lens so formed will absorb stray light near its perimeter but transmit such light near its center.
The prior art describes a process for the preparation of a surface-darkened glass in which a clear glass core is enveloped with a dark glass cladding and both are then subjected to heat and applied pressure to fuse them into a single unit. This prior art process often produces glass objects with imperfections at the interface of the core and the cladding; and, when the surface of such a glass object is polished, pitting and other imperfections often appear at the interface. These surface imperfections cause problems when one attempts to bond the surface to another surface.
It is an object of this invention to provide a process for the preparation of a surface-darkened glass which has substantially fewer surface imperfections and is substantially more uniform than prior art surface-darkened glass;
It is another object of this invention to provide a process for the preparation of surface-darkened glass which does not necessitate the joining of two different glass bodies;
It is yet another object of this invention to provide a process for the preparation of a surface-darkened glass with a relatively low amount of deformation;
It is yet another object of this invention to provide a relatively low-temperature process for the preparation of surface-darkened glass;
It is yet another object of this invention to provide a process for the preparation of surface-darkened glass in which the reaction time is relatively short; and
It is yet another object of this invention to provide a surface-darkened glass object which contains at least about 40 weight percent of silica, elemental antimony, less than about 20 parts per million of halide ion, less than about 20 parts per million of silver ion, less than about 0.01 weight percent of iron, and less than about 0.01 weight percent of ferric ion.